This invention relates to a method of constructing salient pole stator assemblies for dynamoelectric machines. Salient pole motors in various forms have been available for many years. In the past, salient pole motors found general applications in stepper motors. As will be appreciated by those skilled in the arts, stepper motors are used in applications where precise control of rotor location is required. More recently, salient pole motor designs are finding application in both brushless permanent magnet motors and switched reluctance motors.
One of the differences between a brushless permanent magnet motor and a switched reluctance motor concerns the rotor construction. Both switched reluctance and brushless permanent magnet motors include a stator assembly and a rotor assembly. The rotor assembly of the brushless permanent magnet motor has at least one permanent magnet associated with it. Electrical energy is applied to the windings sequentially to drive the motor according to a desired function. The rotor of a switched reluctance motor eliminates the use of magnets. That is to say, it conventionally is constructed from a plurality of individual laminations formed from ferromagnetic material which are joined to one another by conventional means. No magnets are employed with the rotor. Again, the rotor follows the application of electrical energy to the windings according to the predetermined order of that electrical energy application.
Induction motors having distributed windings also have been known since antiquity. Induction motors conventionally have the turns of a winding pole distributed over the predetermined number of teeth of the stator core. For example, the stator core may also be constructed from a plurality of individual laminations. The laminations commonly have a central bore opening formed in them, and radially outwardly extending winding receiving slots which open on the bore. The material between the winding receiving slots are commonly known as teeth. The windings are distributed over three, five, and seven teeth, for example, the distribution permitting improved performance for the induction motor.
Salient pole machines also are employed in conventional dc motors. Salient pole windings have all the turns of the motor pole in one winding group generally spanning a single tooth of the stator core. Both brushless permanent magnet motors and switched reluctance motors operate similarly to a conventional dc motor, except that the physical commutator of a dc motor is replaced by an electronic control circuit. The electronic control circuit provides electrical and commutation to the windings so that the rotor performs in a desired matter.
Construction of a stator assembly for distributed windings and salient pole windings in general have developed along three paths. Induction machines now are commonly wound on machines where the turns of a winding coil are manufactured by extremely high speed machines and transferred, either manually or automatically, to an axial insertion device. In the alternative, the windings may again be wound on high speed winding machines, and transferred to a winding transfer tool for later manual placement on the axial insertion device. Apparatus of the latter type are described in U.S. Pat. No. 3,714,973, the disclosure of which is incorporated herein by reference.
Salient pole winding machines, on the other hand, generally have windings constructed by two methods. The individual poles are wound on plastic bobbins, for example, and the individual bobbins are placed over the teeth of an associated stator assembly. In the alternative, oscillating gun type winders are employed to wind the salient pole directly on the stator core. While both salient pole winding construction methods work for their intended purpose, they are characterized by relatively slow winding speeds, especially when compared to induction motor distributed winding techniques, and have poor slot fills because both the bobbin and the oscillating gun require considerable space in the slots in order to function properly. As will be appreciated by those skilled in the art, the term slot fill, for the purpose of this Specification, means the area of the slot available for the winding divided into the area occupied by the number of turns of the winding in that slot. Induction motor slot fills in the vicinity of slot fills approaching 70% are common. Slot fills for salient pole windings commonly are 40%-50%. While bobbin and oscillating guns also have been employed with more conventional induction motors, axial insertion of for salient pole motors was not considered feasible because salient pole motors, particularly switched reluctance and brushless permanent magnet motors, are generally characterized by relatively long stack heights and short end turn winding designs.
Motor designers have two basic techniques for matching motor performance to a particular intended application of the motor. Motor stack height can be increased while winding turns are reduced because the mix of active material, that is to say, the steel and copper producing the associated motor flux, can be equalized by that technique. Conversely, motor turns can be added and steel removed to accomplish a similar result. Again, as will be appreciated by those skilled in the art, copper unit costs are considerably higher than steel unit costs. Conventional motor design wisdom suggests that adding copper turns and end turn length to a motor design (thereby substituting increasing copper cost for the motor) will not improve overall motor costs. When additional turns were added to motor designs in the past, and the steel or stack height decreased to obtain equivalent performance motors, the motor design choice often occurred because of two reasons: (i) space requirements in a particular application required reduced overall motor length, and (ii) the motor""s original design was so uneconomical that a cost reduction inevitably resulted with the redesign; as opposed to a method of providing a lower cost construction technique of a motor originally designed to proper electrical and mechanical specifications. In general, copper costs far exceed lamination steel costs per pound, so that a reasonable motor designer seeking to achieve a low cost motor construction for a motor having proper design characteristics would not substitute copper for steel in a motor design where low cost is the objective. That is to say, motors in general and salient pole motors in particular are presently designed for economical production because the motors are already designed to work the active material of the motor at or near its electrical capacity. Once an economic design is achieved, it is not logical to believe that the substitution of a high cost material for a low cost material will reduce product cost.
We have found that contrary to conventional motor construction maxims, low cost salient pole motors can be produced (for even previously properly designed motors) by techniques that reduce the stator core active material, either by reducing lamination size, with or without additional winding turns but, in any event, by increasing slot fill for the motor design; or by thereafter substantially increasing the turns per pole, substantially decreasing motor stack height; and constructing the winding externally of the stator assembly, and axially inserting the winding into position on the stator assembly.
One of the objects of this invention is to provide a method of constructing a low cost salient pole motor design.
Another object of this invention is to provide a salient pole motor design which enables the salient poles to be axially inserted into position on the stator core;
Another object of this invention is to provide a salient pole motor having relatively high slot fills;
Another object of this invention is to provide a salient pole motor in which conventional insulation techniques may be employed with the stator core of the motor;
Another object of this invention is to provide a method of constructing a salient pole motor in which the windings are automatically transferred to an axial insertion machine;
Still another object of this invention is to provide a method of constructing a salient pole motor in which the windings are transferred to a winding transfer tool for later positioning on an axial insertion machine.
Yet another object of this invention is to provide improved heat transfer capabilities of salient pole motors employed in applications where positive air flow techniques are not commonly employed.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.
In accordance with this invention, generally stated, a low cost construction method for salient pole motors is unexpectedly provided by reducing the ferromagnetic content of the motor and increasing the magnet wire content of the winding.
In one preferred form of the invention, the electrical performance for the motor in its intended use is determined. Thereafter, the motor design is accomplished by providing a core stack height that approaches the minimum stack height while the slot fill of the windings approaches the maximum slot fill for the lamination design utilized for the stator core. In another preferred form of the invention, the stator core lamination size is reduced while the slot fill for the winding is increased.
In all embodiments, the winding is wound externally of the stator core, and transferred to an axial insertion device. After core placement, the salient pole winding is axially inserted into the core.
In another form of the invention, the winding is wound externally of the motor and placed on a suitable transfer tool, where it is stored until use. The winding thereafter is moved from the transfer tool to the axial insertion device.
In another form of the invention, the winding, after being formed externally of the stator core, is automatically positioned on the axial insertion device.